Precipitation is the process of conversion of a chemical substance
into a solid
from a solution
by converting the substance into an insoluble form or a super-saturated solution
. When the reaction occurs in a liquid solution, the solid formed is called the precipitate. The chemical agent that causes the solid to form is called the ''precipitant''.
Without sufficient force of gravity (settling
) to bring the solid particles together, the precipitate remains in suspension
. After sedimentation
, especially when using a centrifuge
to press it into a compact mass, the precipitate may be referred to as a 'pellet'. Precipitation can be used as a medium. The precipitate-free liquid remaining above the solid is called the 'supernate' or 'supernatant'. Powders derived from precipitation have also historically
been known as 'flowers'. When the solid appears in the form of cellulose
fibers which have been through chemical processing, the process is often referred to as regeneration.
Sometimes the formation of a precipitate indicates the occurrence of a chemical reaction. When barium chloride
solution reacts with sulphuric acid
, a white precipitate of barium sulphate
is formed. When potassium iodide
solution reacts with lead(II) nitrate
solution, a yellow precipitate of lead(II) iodide
The precipitation may occur if concentration of a compound exceeds its solubility
(such as when mixing solvents or changing their temperature). Precipitation may also occur rapidly from a supersaturated
In solids, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching or
, and the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is routinely used to synthesize nanoclusters
An important stage of the precipitation process is the onset of nucleation
. The creation of a hypothetical solid particle
includes the formation of an interface
, which requires some energy
based on the relative surface energy
of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs.
The hydroxide precipitation is the most widely used industrial precipitation in which metal hydroxides
are formed by using calcium hydroxide
(''slaked lime'') or sodium hydroxide
(''caustic'' ''soda'') as the precipitant.
of propionic acid
precipitate on cooling
Precipitation reactions can be used for making pigment
s, removing salts
from water in water treatment
, and in classical qualitative inorganic analysis
Precipitation is also useful to isolate the products of a reaction during workup
. Ideally, the product of the reaction is insoluble in the reaction solvent. Thus, it precipitates as it is formed, preferably forming pure crystals
. An example of this would be the synthesis of porphyrin
s in refluxing propionic acid
. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration:
Precipitation may also occur when an ''antisolvent'' (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may easily be separated by filtration
, or centrifugation
. An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, and the product precipitates. Precipitation is also useful in purifying products: crude bmim-Cl is taken up in acetonitrile
, and dropped into ethyl acetate, where it precipitates.
Another important application of an antisolvent is in ethanol precipitation
, precipitation from a solid solution
is also a useful way to strengthen alloy
s. This process is known as solid solution strengthening
Representation using chemical equations
An example of a precipitation reaction: Aqueous silver nitrate
) is added to a solution containing potassium chloride
(KCl), the precipitation of a white solid, silver chloride
(AgCl), is observed. (Zumdahl, 2005)
:AgNO3 + KCl -> AgCl (v) + KNO3
The silver chloride (AgCl) has formed a solid, which is observed as a precipitate.
This reaction can be written emphasizing the dissociated ions
in a combined solution. This is known as the ionic equation
:Ag+ + NO3^- + K+ + Cl^- -> AgCl (v) + K+ + NO3^-
A final way to represent a precipitate reaction is known as a ''net ionic reaction''.
Many compounds containing metal
ions produce precipitates with distinctive colors. The following are typical colors for various metals. However, many of these compounds can produce colors very different from those listed.
Other compounds generally form white precipitates.
Precipitate formation is useful in the detection of the type of cation
in a salt
. To do this, an alkali
first reacts with the unknown salt to produce a precipitate that is the hydroxide
of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used in sequence – for example, a barium nitrate
solution will react with sulfate
ions to form a solid barium sulfate
precipitate, indicating that it is likely that sulfate ions are present.
Digestion, or ''precipitate ageing'', happens when a freshly formed precipitate is left, usually at a higher temperature
, in the solution from which it precipitates. It results in cleaner and bigger particles. The physico-chemical process underlying digestion is called Ostwald ripening
Precipitation reactions of certain cationsDigestion InstrumentsA Thesis on pattern formation in precipitation reactions